Balanced modulators such as Gilbert cell mixers are adversely affected by electronic noise. In particular, in-phase/quadrature (I/Q) modulator designs based on Gilbert cell mixers have degraded performance when exposed to electronic noise that may generate spurious signals outside a desired frequency band. A significant source of this out of band electronic noise is shot noise that is proportional in magnitude to a direct current (DC) supplying the I/Q modulator with power. The shot noise of most concern is typically generated in the I/Q modulator's commutating transistors and baseband transconductor stage.
Prior art I/Q modulator designs based on Gilbert cell mixers typically use a class A transconductor stage. Examples of such transconductor stages include a degenerated differential transistor pair amplifier and a linearized feedback amplifier with a class A current mode output stage. However, the use of a class A transconductor stage as part of an I/Q modulator is problematic from a shot noise perspective because a DC current supplying the class A transconductor stage is dictated by a worst-case peak current requirement. In this case, the worst-case peak current requirement is a function of the worst-case peak-to-average-ratio (PAR) for current rather than being a function of an adjacent channel leakage ratio (ACLR) or other noise consideration. In particular, the DC current supplying the class A transconductor stage must be sufficient to allow a maximum PAR demanded by modulating baseband signals inputted into the class A transconductor stage. For example, if the instantaneous signal current required from the class A transconductor stage exceeds the available DC current, the class A transconductor stage will suddenly deliver a nonlinear output that will cause spectral splatter along with a relatively rapid degradation in ACLR.
Moreover, with telecommunication modes such as Wideband Code Division Multiple Access (WCDMA), High-Speed Uplink Packet Access (HSUPA), and Code Division Multiple Access (CDMA), PAR is not well defined and may vary between a range of 3 dB and 11 dB depending on the number of control and data channels being transmitted. Thus, the direct current supplying the class A transconductor stage is dictated by the worst-case peak current requirement, which in this case is 11 dB. Therefore, a relatively high quiescent current is set for the overall I/Q modulator design. As a result of this relatively high quiescent current, undesirable shot noise is generated together with a waste of power. Accordingly, there remains a need for a low noise linearized and efficient transconductor stage that is usable with active I/Q modulator designs based on Gilbert cell mixers.